This invention relates to a pneumatic radial tire. More particularly, this invention relates to a pneumatic radial tire which can reduce a production cost while keeping tire performance at the same level as that of prior art tires, and particularly to a pneumatic radial tire suitable for passenger cars.
Generally, pneumatic radial tires are equipped with at least one carcass layer formed by arranging reinforcing cords in a tire radial direction inside a tire as a tire skeletal structure, and this carcass layer keeps a tire internal pressure. At least two belt layers, each of which is formed by arranging reinforcing cords in such a fashion that their inclining directions with respect to a tire circumferential direction cross one another in mutually opposite directions between the layers, are disposed on the outer circumferential side of the carcass layer of a tire tread portion. These belt layers protect the carcass layer from an impact from a road surface and an external damage, and function as a xe2x80x9choopxe2x80x9d keeping rigidity in the tire circumferential direction.
Recently, price competition has become severer and severer in the tire industry, too, and the pneumatic radial tire described above must be produced essentially at a low cost without spoiling conventional tire performance.
It is therefore an object of the present invention to provide a pneumatic radial tire which can attain a lower cost of production while keeping tire performance at the same level as that of conventional pneumatic radial tires.
The pneumatic radial tire according to the present invention for accomplishing the object described above comprises at least one carcass layer formed by arranging reinforcing cords, both end portions of which are turned up from the inside to the outside of the tire round right and left bead cores, respectively, and at least two belt layers formed by arranging reinforcing cords in such a manner that their inclining directions with respect to a tire circumferential direction cross one another in mutually opposite directions between the layers and disposed on the outer circumferential side of the carcass layer of a tread portion, characterized in that a carcass strength coefficient K of the carcass layer defined by the following formula (1) is set so as to satisfy a value of 0.15 to 0.35 N/mmxc2x7kPa at portions on the tread portion center side of positions P which are away by 10% of a belt width of a belt layer having the second greatest width from edges thereof towards its inside, and a value of at least 0.5 N/mmxc2x7kPa at portions ranging from the positions P to bead portions:
K(N/mmxc2x7kPa)=[number of reinforcing cords arranged (pcs/mm)]xc3x97[strength of reinforcing cords (N)]xc3x97[number of carcass layers]÷[maximum air pressure (kPa)]xe2x80x83xe2x80x83(1)
The inventors of the present invention have carried out intensive studies on radial tire structures whose strength design is made on the basis of a maximum air pressure, and have specifically noted that because the belt layers are disposed at the tread portion, the tire internal pressure is kept by the carcass layer and the belt layers in which they bear their share of that. Therefore, the present inventors have realized that the strength of the carcass layer can be reduced at least in the area of the tread portion and a part of the tire internal pressure can be shared by the belt layers.
In other words, in the pneumatic radial tires according to the prior art, the carcass layer must bear the internal pressure at sidewall portions, too, so that the carcass strength has been designed to the strength necessary for reinforcing the sidewall portions. At the tread portion, however, the belt layers bear a part of the tire internal pressure and for this reason, the carcass strength has been greater than necessary in the conventional pneumatic radial tires.
Besides the observation described above, the present inventors have paid a specific attention to the carcass strength coefficient K which is defined by the formula (1) and falls within substantially the same range even when the kinds of the tires are different. As to the two layers of the belt layers formed by so arranging the reinforcing cords as to cross one another, therefore, the present inventors have found that when the carcass strength coefficient K is calculated, the belt layers can bear a strength corresponding to 0.35 N/mmxc2x7kPa. Accordingly, in the carcass layer at the tread portion where the belt layers bear a part of the tire internal pressure, the carcass strength coefficient K need not be set to 0.5 N/mmxc2x7kPa or more as has been required in the prior art tires, and it has been found out that tire durability can be secured sufficiently even when the value is lowered to the minimum value of 0.15 N/mmxc2x7kPa by reducing the number of arrangement of the reinforcing cords (end number), etc. It has been found out also that in order to improve the production cost, the upper limit of the carcass strength coefficient K must be set to 0.35 N/mmxc2x7kPa. Further preferably, the coefficient K is within the range of 0.2 to 0.3 N/mmxc2x7kPa.
Therefore, in the sidewall areas where a part of the tire internal pressure cannot be borne by the belt layers, the carcass strength coefficient K is set to at least 0.5 N/mmxc2x7kPa in the same way as in the prior art tires. The upper limit of this carcass strength coefficient K is preferably 1.5 N/mmxc2x7kPa. Even when the carcass strength coefficient K is increased beyond this upper limit, the carcass strength becomes excessive as far as tires for passenger cars in general are concerned and such a strength is meaningless but merely increases the tire weight. However, the carcass strength coefficient K must not be decreased below 0.35 N/mmxc2x7kPa in the entire area occupied by the belt layers, and the value of the carcass strength coefficient K of at least 0.5 N/mmxc2x7kPa must be maintained at portions up to positions P which are away by at least 10% of the belt width from the edges of the belt layer having the second greatest width toward inside, in the same way as in the sidewall areas. On the other hand, the area in which the carcass strength coefficient K is set to 0.15 to 0.35 N/mm kPa at the tread portion as described above need not be the area corresponding to the belt layers, but may be the area on the tire center side of the positions P described above.
As described above, the carcass strength coefficient is smaller than in the prior art tires at the tread portion at which the belt layers bear a part of the tire internal pressure and consequently, the number of arranged reinforcing cords of the carcass layer, the diameter of the reinforcing cords, the number of the carcass layers, etc, can be reduced. Therefore, the material cost can be reduced, and the reduction of the production cost as well as the tire weight can be accomplished.
Moreover, even when the materials of the carcass layer are reduced as described above, carcass durability required at the tread portion can be secured, and the drop of tire performance does not occur.